1. Field of the Invention
The present invention relates to a laser scanning unit, and more particularly, to a laser scanning unit to improve speed and compactness of a laser printer.
2. Description of the Related Art
Generally, a laser printer reproduces an image by focusing a laser beam from a laser diode onto a photosensitive drum 20 (see FIG. 1) with respect to an image signal, and transferring an electrostatic latent image from the photosensitive drum 20 to a printing medium such as paper. Such a laser printer has a scanning unit to generate and focus the laser beam onto the photosensitive drum 20.
FIG. 1 is a schematic view showing the structure of a conventional laser scanning unit. Referring to FIG. 1, the conventional laser scanning unit includes a laser diode 10, serving as a light source, by releasing a laser beam, and a collimator lens 11 to make the laser beam from the laser diode 10 parallel with respect to a light axis of the laser beam. The conventional laser scanning unit further includes a cylinder lens 12 to make the parallel laser beam from the collimator lens 11 a linear beam horizontal with respect to a sub-projection direction B, a polygon mirror 13 to move the horizontal linear laser beam at a uniform linear velocity for scanning, and a polygon mirror-driving motor 14 to rotate the polygon mirror 13 at a constant velocity. The conventional laser scanning unit further includes an f-θ lens 15 having a constant refractivity with respect to the light axis, to focus the light beam on a scanning surface by polarizing the light reflected from the polygon mirror 13 in a main-scanning direction A and then by a difference compensation, a reflecting mirror 16 to form a latent image on the surface of the photosensitive drum 20 by reflecting the laser beam from the f-θ lens 15. The conventional laser scanning unit also includes a horizontal synchronization mirror 17 to reflect the laser beam from the f-θ lens 15, in a horizontal direction, and a photo sensor 18 to receive and synchronize the laser beam reflected from the horizontal synchronization mirror 17.
In the conventional laser scanning unit described above, the laser diode 10 irradiates a laser beam corresponding to the image signal of an image, and the laser beam is converted into a parallel ray by the collimator lens 11. The parallel ray is focused on the surface of the polygon mirror 13 on the sub-projection surface by the cylinder lens 12. The light characteristics of the main-projection surface are maintained uniform. Here, the ‘main-projection surface’ is the plane that is in a vertical relation with respect to the rotational axis X of the polygon mirror 13, while the ‘sub-projection surface’ is the plane that is in a vertical relation with respect to the main-projection surface. The light reflected from the polygon mirror 13 is formed into a latent image corresponding to the desired image, as the light is passed through the f-θ lens 15, formed into a predetermined shape on the main and sub-projection surfaces, and focused on the photosensitive drum 20.
The process of forming an image of a line on the main-projection surface will now be described. The laser beam passes through the collimator lens 11 and the cylinder lens 12, and reaches the polygon mirror 13. The laser beam reflected from the polygon mirror 13 is then incident on the f-θ lens 15. Then the laser beam is made incident on the photosensitive drum 20 at a predetermined angle varying according to a facial angle of the polygon mirror 13. That is, the polygon mirror 13 connected with the polygon driving motor 14 is rotated at a predetermined velocity, varying the angle of the incident laser beam to make the laser beam incident on the photosensitive drum 20. As a result, the laser beam is formed on the main-projection surface on the photosensitive drum 20 in the form of a line. The image in the sub-projection direction B is formed as the photosensitive drum 20 is rotated to arrange the line image in the main-projection direction A at predetermined uniform intervals. At this time, to obtain the line images of acceptable quality, the starting points of the respective line images can be aligned constantly by detecting the laser beam reflected from the horizontal synchronization mirror 17 with the photo sensor 18 and then synchronizing the laser beam.
However, the conventional laser scanning unit constructed as above has the following problems. First, in order to obtain a quality image, the structure of the f-θ lens 15 is complicated, making the unit less compact. Furthermore, since the rotational velocity of the polygon mirror-driving motor 14 must be increased in order to perform the printing process at a rapid speed, the manufacturing cost increases.